1. Field of the Invention
The present invention relates generally to apparatus for welding plastics, and more specifically to apparatus for forming seams in sheet materials having weldable plastic components.
2. Description of the Related Art
Apparatus for welding seams in plasticized sheet materials generally include several basic components: infeed guides; a heating element; pressure rollers; and, a drive system for moving either the sheet material or the welder with respect to the other.
Sheets of weldable material are commonly joined to one another with either of two primary seam configurations: the lap weld; or, the fin weld (the fin weld also being known as the "prayer" weld). A third primary seam type, used in binding a sheet's edge to itself, is the hem weld. Variations on, or specific applications of, these three primary seam configurations can be used to create a range of different structures adapted to particular purposes, including; taped reinforcements; taped, butt-welded joints; sleeves; tubes; and, flaps. And, weldable sheet materials can be mated with other materials and structures to yield features useful in the arts to which sheet materials are applied. Examples include hem tubes through which ropes, chains, cables and the like pass for supporting and suspending sheets; and, hem cords (i.e. hem tubes with tightly-bound, semi-rigid rods inside) for anchoring sheet edges to mating channels cut in support structures.
However, currently available seam welders are unable to produce all of the three primary seam types, or are otherwise lacking features which would make them equally practical for creating all three types in an industrial setting. For example, U.S. Pat. No. 4,950,352 issued to Greller in 1990 shows a welder with its pressure rollers oriented for unlimited material widths distal to the rollers with respect to the chassis; but, Greller's chosen roller suspension leaves limited space between the rollers and the chassis, making it impractical for use in lap welding very wide sheets of material. Further, Greller's 1990 welder, being a "hot-air" welder, would suffer limitations inherent to that genus, i.e. being useful only for sheet materials of limited thickness, and being generally noisy.
U.S. Pat. No. 4,855,004 issued to Chitjian in 1989 shows a power-driven, hand-guided, hot-air welder adapted for lap welding seams of wide sheets of material on roof decks, and the like. Lap seams are the primary intended product of the Chitjian welder; fin and hem welds appear technically possible, but less practical for the lack of appropriate infeed guides. Further, being hand-guided, drawbacks of the Chitjian welder include the non-uniformity in welds caused by slight drive speed and direction changes, and great dependence upon the skill of the operator, as well as the previously-mentioned problems with hot-air welders.
Ellenberger, et al, in U.S. Pat. No. 4,774,855 issued in 1988 shows another lap-seam welder adapted for hand-guiding over surfaces covered in weldable sheeting. This is a "hot-wedge" welder, employing a wedge-shaped, high-temperature, electrically-powered metal heating element. Hot-wedge welders such as that shown in the Ellenberger, et al 1988 patent are generally more versatile and reliable in their ability to weld materials of different thicknesses. However, the welder of the Ellenberger, et al 1988 patent is limited by its infeed guide structure in the types of welds it can perform; lap welds are primarily provided-for, while fin and hem welds may be technically feasible to a limited, albeit impractical, extent.
Taped reinforcements and taped, butt-welded joints are the product of the radiant-heat, tape welder disclosed in U.S. Pat. No. 4,913,772 issued to Taylor, et al in 1990. These are lap weld variants are difficult to perform with some other welders. However, the welder of Taylor, et al lacks the infeed guide structure to permit welds of the other primary types.
Greller, et al, in U.S. Pat. No. 5,091,038 issued in 1992, discloses a hot wedge welder with guides for performing lap and hem welds, and fin welds also appear possible. The Greller 1992 welder is "automatic," i.e. self-propelled, and can therefore be run along a sheet's edge under its own power at a precise speed for an accurate and consistent weld. However, the limitations of the chassis construction, and especially the dimension of the arm upon which the hot wedge is mounted on the Greller, et al 1992 apparatus, make lap welding therewith cumbersome, and practical in only limited applications. Specifically, only when both sheets to be lap welded can be laid out distal to the rollers and chassis can extremely wide sheets be lap welded together, because the space between the rollers and chassis is limited. Thus, the Greller 1992 welder cannot be used for lap welding or taped butt-welding wide sheets laid out on a roof deck, for example. Further, Greller, et al's 1992 apparatus appears to be exceedingly dangerous in that its hot wedge, which operates at temperatures of 1000.degree. F., or more, is not shielded to protect an operator from severe burns; and yet further, it projects menacingly from the apparatus when swung away from its welding position.
European Patent Application No. 79890022.1 filed by Winkler, et al in 1979 discloses a hot wedge welder illustrated as being capable of lap and fin welds, and which may have limited hem welding capability. The Winkler, et al welder employs an S-shaped chassis upon which opposed pressure rollers appear able to be mounted on either horizontal or vertical axes for either lap or fin welding, respectively. However, switching between roller orientations appears cumbersome, and would make precise centering of the hot wedge in the plane tangent to the point of roller occlusion difficult, wedge centering being quite important to the quality of the weld.
Methods and apparatus heretofore employed for constructing hem cords and for inserting ropes of cables in hem tubes are also inadequate. Most commonly, the hem is first constructed, and then a rope, cable or semi-rigid rod is threaded through the hem tube by hand. This task becomes exceedingly difficult when the hem tube is very long. And, even when a seam sealing machine's chassis includes a relieved area permitting a chain, rope or cable to be bound directly into the hem tube as the hem is being made, these machines are not adequate for making good quality hem cords because the hem tubes constructed therewith are characteristically too loose on the semi-rigid rod, allowing it to slip around more than is desirable.
Thus, it appears a need exists for a safe and reliable seam welder able to construct lap, fin and hem welds, as well as taped reinforcements, taped butt-welded joints, sleeves, tubes, hem cords and, flaps, while being minimally limited in the thickness of material it can weld. And, complexity and imprecision in switching among these different weld-types should be minimized, as well. Further, an ideal welder would be able to perform lap and taped butt-welds without limitations on the width of either sheet being welded. Yet further, accurate wedge centering to assure uniform heating of the sheets being welded would be desirable, as would precise speed control to assure that the weld is consistent along its length. And, means for producing a tight, continuous, unlimited-length hem cord would also be provided.